Log the reservoir: volume, days since the last complete change, and the EC drift per day your meter actually records.

Model the batch EC: fresh 1.8 + drift × days, plus the small salt term accumulation ÷ 1000 × 0.02 × days ÷ biofilm — drift dominates, salt stays a quiet second order.

Model the pH line: fresh 6.0 plus pH-drift × days — the 5.5–6.5 band the archived journal logged drifts out predictably.

Count down the change: average the 0.5 mS/cm EC budget and the 1.0 pH rise against what is already used, then add the biofilm factor so a slime-prone room changes earlier.

Size the flush: 0.2 × reservoir plus 0.5 × salt buildup, trimming what your top-up habit already cycled, and watch the biofilm composite and the ORP decay line.

Close the decision: the green (>7 days) yellow (3–7) red (<3, biofilm >7, pathogen >6, ORP <400) band and a schedule cadence for the grow.